Bug:

NextJs app with basePath results in assets 403'ing

How to reproduce

run npx sst deploy on this repo

https://github.com/willwashburn/sst-basepath-assets-issues

load the index page. assets get 403'd

Expected

With basePath: "/example-base-path", these should resolve:

• GET /example-base-path/_next/static/... -> 200
• GET /example-base-path/favicon.svg -> 200

Actual

The HTML references base-path-prefixed assets, but only root asset paths work:

• GET /example-base-path/_next/static/... -> 403
• GET /_next/static/... -> 200
• GET /example-base-path/favicon.svg -> 403
• GET /favicon.svg -> 200

Root Cause

This looks like an SST CloudFront router bug.

OpenNext v3.9.16 emits base-path-aware S3 copy metadata:

https://github.com/opennextjs/opennextjs-aws/blob/v3.9.16/packages/open-next/src/build/generateOutput.ts#L198-L208

But SST’s router strips the base path only for matching, then reuses the original request URI when rewriting to S3

So a request like:

• /example-base-path/_next/static/chunks/app.css

matches using:

• /_next/static/chunks/app.css

but gets rewritten to S3 as:

• /_assets/example-base-path/_next/static/chunks/app.css

instead of:

• /_assets/_next/static/chunks/app.css